1. Field of the Invention
The control of vibrations in composite structures is an important area of research in aerospace, automotive and other industries. For example, spacecraft vibrations initiated by attitude adjusting thrusters, motors and thermally induced stresses inhibit accurate aiming of antennas and other equipment carried by the craft. Such vibrations can cause severe damage to the craft and its associated equipment. Fatigue failure of structural components can occur at stresses well below static load limits.
Composite materials have been used to construct a wide variety of structural elements, including tubes, enclosures, beams, plates and irregular shapes. Objects as diverse as rocket motor housings and sporting goods, notably skis, archery arrows, vaulting poles and tennis rackets have been structured from composite materials. While composite constructions have offered many significant advantages, such as excellent strength and stiffness properties, together with light weight, the poor vibration damping properties of such constructions have been of concern.
The invention relates to composite material structures having increased damping with little or no sacrifice of structural stiffness or strength.
The invention also relates to the methods and apparatus for manufacturing the aforementioned composite material structures.
Another aspect of the invention is directed toward the fabrication of a wavy fiber pregreg (fibers preimpregnated with epoxy resin). Such prepregs not only have an aesthetic appeal but also may be fabricated with selected variable volume fractions to accommodate a variety of applications.
2. Description of Related Art
The following terms used herein will be understood to have their ordinary dictionary meaning as follows:
Fiber: a thread or a structure or object resembling a thread. a slender and greatly elongated natural or synthetic filament. (This definition includes metal fibers).
Matrix: material in which something is enclosed or embedded.
Viscoelastic: having appreciable and conjoint viscous and elastic properties.
Lamina(e) a thin plate . . . : LAYER
Composite: made up of distinct parts.
It may be further helpful to note the definitions of G0 and G1 geometric continuity. xe2x80x9cIf two curve segments join together, the curve has G0 geometric continuity. If the directions (but not necessarily the magnitudes) of the two segments"" tangent vectors are equal at a join point, the curve has G1 geometric continuity. G1 continuity means that the geometric slopes of the segments are equal at the join point.xe2x80x9d Foley et al. Computer Graphics Principles and Practice, Addison-Wesley, 1996, p. 480.
The article xe2x80x9cUnderstanding Vibration Measurements,xe2x80x9d George F. Lang, Sound and Vibration, March, 1976, p. 26, presents a generally accepted mathematical treatment of the vibration of mechanical structures under environmental loading. This treatment applies generally to composite structures, and is incorporated by reference for purposes of this disclosure for its explanation of the amplification factor Q and its relationship to the viscous damping factor xcex6. The xe2x80x9closs ratioxe2x80x9d referred to in this disclosure is twice the viscous damping factor as defined by Lang.
Conventional methods used to control the often destructive levels of vibration take many forms, from simple passive treatments to extensive redesign of structures. One of the simplest and often most effective passive damping treatments involves the use of thermo-visco-elastic (TVE) materials. TVE materials such as 3M""s Scotchdamp series (ISD-112 is one example), exhibit dissipative qualities which make them useful in a number of passive damping treatments. Some of the first uses of TVE materials to increase structural damping involved the use of surface patches of aluminum foil and viscoelastic adhesives. These conventional approaches to surface damping treatments are called constrained or embedded-layer damping, and produce modest gains in damping over undamped structures.
One of the more common passive damping methods, xe2x80x9cconstrained layer dampingxe2x80x9d or CLD is discussed in the article xe2x80x9cDamping of Flexural Waves by a Constrained Viscoelastic Layer,xe2x80x9d Kerwin, Journal of the Acoustical Society of America, 1959, Vol. 31, Issue 7, pp. 952-962. According to Kerwin, CLD is achieved by bonding a thin layer of metal sheet, usually aluminum, to an existing structure with a viscoelastic adhesive. According to this technique, damping material, typically a viscoelastic material, is applied to the surface of a composite structure, such as an airplane wing. The damping material is sandwiched between the composite surface and a rigid layer, such as a thin aluminum sheet. This approach has generally been remedial in character and is accomplished at the sacrifice of other considerations, such as weight, aesthetics and ideal surface configuration. Shear strains are developed in the viscoelastic material when the original structure bends or extends. Damping occurs when the deformation of the viscoelastic adhesive creates internal heat in the viscoelastic material and dissipates energy.
Compared to an undamped structure, this approach is modestly successful but its effectiveness decreases markedly as the ratio of the thickness of the base structure to the thickness of the viscoelastic material increases. Thus, surface treatments alone cannot provide significant levels of damping to structural members where greater strength and stiffness are important. In the article xe2x80x9cUse of Strain Energy Based Finite Element Techniques in the Analysis of Various Aspects of Damping of Composite Materials and Structures,xe2x80x9d Hwang, et al., Journal of Composite Materials, 1992, Vol. 26, Issue 17, pp. 2585-2605, this problem was reported, and it showed that the advantage of aluminum foil viscoelastic constrained layer damping was eclipsed by the inherent damping in conventional composites when the required thickness of the structure exceeded about three tenths of an inch. The authors determined that a xc2x145xc2x0 graphite/epoxy composite provided approximately uniform damping of about 1.5% in thick sections.
It is known that laminated beams composed of alternating layers of elastic and viscoelastic materials can dissipate vibratory energy while maintaining a degree of structural integrity. The article xe2x80x9cComposite Damping of Vibrating Sandwich Beams,xe2x80x9d DiTaranto, et al, Jour. of Engineering for Industry, November, 1967, p. 633, presents a theoretical description of such structures.
The feasibility of co-curing embedded layers of damping materials in a composite structure has been demonstrated. See, for example, Rotz, Olcott, Barrett, xe2x80x9cCo-cured Damping Layers in Composite Structures,xe2x80x9d Proceedings 23rd International SAMPE Technical Conference, Vol. 23, pp. 373-387, 1991. Vibration control can thus be designed into a structure prior to its actual construction. Composite tubes have been constructed from a pair of concentric composite stiffness layers, the annular space between them being occupied by a damping layer. It is known that when fiber-reinforced materials are loaded along any axis not parallel or perpendicular to the fibers, shear deformations are induced. A tube with plies oriented xe2x80x9coff-axisxe2x80x9d (with respect to the central axis of the tube) will twist when loaded axially. With the plies of the two stiffness layers oriented at opposite but similar angles with respect to the tube axis, intense shear deformation is induced in the. damping layer when the tube is loaded axially. The stiffness of the tube remains high because the load passes only through the stiffness layers, and the damping layer adds little weight to the structure. Unfortunately, the most significant shear displacements occur at the free ends of the tube. Constraining the rotational deformations of the stiffness plies at either end of the tube eliminates any shear deformations in the damping layer at that end, thereby reducing the damping effect of the system. Complicated end fixtures are thus required to allow the requisite free end displacements while still transmitting axial loads.
Stress-coupled co-cured composite viscoelastic structures are formed when layers of uncured fiber composites and TVE materials are alternately stacked together and co-cured in an oven. These structures provide impressive levels of damping and can be categorized by the fiber orientation methods used to induce damping in the TVE material.
The article xe2x80x9cA Design for Improving the Structural Damping Properties of Axial Members,xe2x80x9d Barrett, Proceedings of Damping, 1989, Vol. HBC-1-18, proposed designs using conventional angled-ply (xc2x1xcex8) composite lay-ups of straight fiber pre-preg materials (e.g., fabric layers preimpregnated with resin material). Barrett used the inherent shear coupling properties of composite materials to design damped composite tubular components which would induce significant damping in the structure. Fiber-reinforced composites will shear as the fibers attempt to align themselves with the applied loads when these loads are not parallel or perpendicular to the fiber. Because of this behavior, a plate constructed with a positive xcex8 fiber layer, a viscoelastic material layer, and a negative xcex8 fiber layer will generate large shear strains (xcex3) in the TVE material when an axial load is applied to the tube. Barrett""s research showed that maximum shearing was experienced at the ends of the tubes. However, connections at the tube ends eliminated much of the damping effect, rendering the design impractical for many applications.
U.S. patent application Ser. No. 07/780,923 to Olcott et al., filed Oct. 22, 1991, proposes xe2x80x9cstress coupled dampingxe2x80x9d (SCD) which also uses conventional angled ply (xc2x1xcex8) composite lay-ups of straight fibers, but abruptly changes the fiber orientation several times throughout the structure. According to Olcott et al., each composite layer is comprised of multiple segments of pre-preg composite material. The patent teaches the use of adjacent segments, having fibers therein which cumulatively form a chevron pattern. The composite layer may include several pre-preg plies in which the segments are staggered or overlapped to strengthen the joint. This creates a region in the composite layer that exhibits quasi-isotropic properties.
By selecting the fiber angle, thickness, and segment lengths, significant shearing in the viscoelastic layers was observed over the entire structure, not just at the ends as in Barrett""s design. As a result, the structure retained high stiffness and exhibited improved damping even when the ends were clamped. The following publications, incorporated herein by reverence, are cited for further details on this subject.
1. Olcott, D. D., xe2x80x9cImproved Damping in Composite Structures Through Stress Coupling, Co-Cured Damping Layers, and Segmented Stiffness Layers,xe2x80x9d Brigham Young University, Ph.D. Thesis, August 1992.
2. Trego, A., Eastman, P. F., Pratt, W. F., and Jensen, C. G., xe2x80x9cReduced Boring Bar Vibrations Using Damped Composite Structures,xe2x80x9d Proceedings of the 1995 Design Engineering Technical Conferences, Vol. 3, Part A, pp. 305-311.
3. Trego, A. and Eastman, P. F., xe2x80x9cOptimization of Passively Damped Composite Structures,xe2x80x9d International Journal of Modelling and Simulation, Vol. 17, No, 4, 1997;
4. Trego, A., Olcott, D. D., and Eastman, P. F., xe2x80x9cImproved Axial Damping of Mechanical Elements Through the Use of Multiple Layered, Stress Coupled, Co-Cured Damped Fiber Reinforced Compositesxe2x80x9d, Journal of Advanced Materials, January, 1977, p.28.
The text Vibration Damping of Structural Elements, by C. T. Sun and Y. P. Lu, Prentice Hall PTR, 1995, presents a discussion of fiber reinforced damping in composite materials using viscoelastic materials for damping, and is incorporated herein by reference.
Although damping was improved, such structures manufactured by conventional lay-ups of straight fiber segments were time consuming to make, error prone, and could not be readily automated. The use of composite struts to dampen vibrations has been proposed in U.S. Pat. No. 5,203,435 to Dolgin. According to Dolgin, plies with opposing chevron patterns of fibers would convert longitudinal vibrational stresses into shear stresses in an intermediate viscoelastic layer which would then dissipate the vibrational energy. One specific structure proposed is a tube with an inner ply oriented at 0xc2x0, a second layer oriented at +45xc2x0, a central viscoelastic layer, a subsequent layer oriented at xe2x88x9245xc2x0, and an outer layer oriented at 90xc2x0. Dolgin also illustrates a sinewave pattern. The disclosure of the Dolgin patent is incorporated herein by reference.
The materials, methods and applications of composites manufacturing are set forth in detail in the literature. An exemplary source for information of this kind is the textbook xe2x80x9cFUNDAMENTALS OF COMPOSITES MANUFACTURINGxe2x80x9d by A. Brent Strong, published by The Society of Manufacturing Engineers, Dearborn, Mich. is 1989. The disclosure of this textbook is incorporated herein by reference for its explanation of the art of composites manufacturing as it is currently practiced. Similarly, the textbook xe2x80x9cVISCOELASTIC PROPERTIES OF POLYMERS,xe2x80x9d John D. Ferry, Wiley, New York, (3rd Ed., 1980) presents a thorough summary of the nature, behavior and identity of typical materials which may be selected by composites technicians for use as damping materials. A good discussion of viscoelasticity as it applies to materials of interest to this disclosure is presented in the publication xe2x80x9cLinear Viscoelasticity,xe2x80x9d Driscoll, et al., available from the Structural Products Department of 3M Company, St. Paul, Minn.
There remains a need for a composite structure capable of diverse configuration with improved damping characteristics and which avoids the limitations of the structural approaches heretofore suggested for use with composites materials.
The present invention is directed to xe2x80x9ccontinuous wave composite viscoelasticxe2x80x9d (CWCV) structures, as well as the methods and apparatus of manufacturing them.
The invention is also directed to a CWC (continuous wave composite) which forms a continuously wavy prepreg for use with or without a separate viscoelastic layer.
In both the CWCV and CWC structures, the wavy characteristic of the fiber is optimally varied in at least one of a period, amplitude or shape characteristic.
In accordance with another aspect of the invention, there is provided a fiber reinforced viscoelastic tape which may be used in many diverse applications.
The lay of fiber in a CWCV composite layer is varied continuously in a periodic wavelike form. A simple sinusoid wave form may be used, however, other wave forms which may or may not be periodic may also be used. It is also envisioned to employ an optimal wave form for damping particular vibration frequencies at particular locations of a structure.
The terminology CWC (continuous wave composite) will be used to defined any fiber-matrix combination having at least one fiber without a break (or interruption) and having a pattern which can be defined by a mathematical algorithm. Typically, such curves have G1 geometric continuity. A fourier series expansion is a mathematical algorithm which can, in general, be used to define nearly any desired shape such as pseudo random, square wave, straight line, triangular wave or any of the shapes shown in FIGS. 1-6 below.
The terminology CWCV (continuous wave composite viscoelastic) will be used to defined a composite structure which uses at least one layer of CWC material having viscoelastic properties (or xe2x80x98anisotropic viscoelasticxe2x80x99); or at least one layer of CWC material combined with at least one layer of viscoelastic material either in a sandwich construction or adjacent construction.
A CWCV is defined by specifying the angle of the fiber lay along the composite layers (e.g. the orientation angles of the fiber with respect to the loading direction), the thickness of the composite layers, and the number of composite and viscoelastic layers in the structure.
The ends of a CWCV structure according to the present invention may be restrained without significantly reducing the overall damping properties of the structure. There results a structural element possessing high axial stiffness and low weight. The structural elements of this invention offer markedly superior damping capabilities but are nevertheless useable with simple attachment fixtures and methods.
The composite structures of this invention may take a variety of forms, including tubes, plates, beams or other regular or irregular shapes. In any event, a typical structure will at a minimum include a first stiffness layer or matrix, a damping material, and a second stiffness layer or matrix. Each stiffness layer or matrix will include at least one reinforcing fiber and will be at least several thousandths of an inch thick. Layers with multiple plies and of much greater thickness; e.g. several inches, are envisioned. The fibers of a multi-ply layer may be of similar or dissimilar orientation. The damping material may be of any appropriate thickness, depending upon the application involved, as well as the properties of the damping material. selected. The damping material may comprise another layer interposed between the stiffness layers, or may be incorporated into the stiffness layer. Typically, the damping material will be as thin as is practical, to avoid adding excess weight to the structure. It is not unusual, however for a layer of damping material to exceed in thickness the total thickness of the stiffness layers. The stiffness layers may be constructed of any of the reinforcing fibers and matrix materials which would otherwise be appropriate for a particular application. The damping material will ordinarily be selected to provide optimum damping loss at the temperatures and vibrational frequencies expected to be encountered by the composite structure.
With two stiffness layers, shear strain occurs between the two stiffness layers, e.g. in an intermediate damping layer. Other embodiments contemplate the use of multiple damping layers. For example, the use of three stiffness layers permits the exercise of two intermediate damping layers, with greater damping effect. The stiffness layer may be combined with the damping layer by utilizing a viscbelastic material as all or a portion of the impregnating resin. Co-curing of the stiffness and damping layers simplifies the construction of both complex structures and structural components.
One embodiment of the invention may be characterized as a structure for use as a pre-preg comprising a fiber tow, and a resin matrix containing the fiber tow, wherein the fiber tow is held in the resin in a waveform varying in at least one of a period, amplitude or shape characteristic. The prepreg may be utilized as at least part of a support structure and in such a case, the fiber tow may have a first waveform characteristic selected to dampen a first vibration mode of the support structure and a second waveform characteristic, superimposed on the first waveform, wherein the second waveform characteristic is selected to dampen a second vibration mode of the support structure. The first waveform characteristic may be one of a period and amplitude characteristic and the second waveform characteristic may also be one of a period and amplitude characteristic.
Another embodiment of the invention may be characterized as a structure comprising a fiber tow, a resin matrix containing the fiber tow, and a viscoelastic material adjacent to or contacting the fiber tow, wherein the fiber tow is held in the resin in a waveform varying in at least one of a period, amplitude or shape characteristic.
Another embodiment of the invention may be characterized as a material for damping vibration, the material having a loading direction and comprising a first matrix containing a first set of fibers, the first set of fibers having a first waveform varying in at least one of a period, amplitude or shape characteristic along the loading direction; a second matrix containing a second set of fibers, the second set of fibers having a second waveform; and an viscoelastic material positioned between the first matrix and the second matrix. The first and second waveforms may be different from one another, and they may differ only in their phase angle relative to one another. Further, the second waveform may vary in at least one of a period, amplitude or shape characteristic along the loading direction, and at least one of the first and second sets of fibers may be at least partially contained within the viscoelastic material. The fibers of the first and second sets may be relatively oppositely oriented along the loading direction.
The CWC material is also characterized in that the volume fraction changes in such a way that the first set of fibers comprises a plurality of first fibers relatively laterally spaced with respect to the loading direction, wherein the relative lateral spacing is inversely proportional to the angular orientation with respect to the loading direction.
Another embodiment of the invention may be characterized as a method of fabricating an impregnated material comprising the steps of (a) passing a plurality of fibers to at least one first roller, the first roller contacting the plurality of fibers for moving same along a first direction, (b) passing the plurality of fibers to at least one second roller, the second roller contacting the plurality of fibers for moving same along the first direction, (c) transversely relatively moving at least one of the first and second rollers in a second direction while each of the first and second rollers contacts the plurality of fibers thereby moving the plurality of fibers generally perpendicular to the first direction, and (e) applying a carrier to the fibers as the fibers pass proximate the second roller.
The applying step may comprise applying an uncured polymer matrix to the fibers as the fibers pass proximate the second roller.
The applying step may comprises applying a viscoelastic material to the fibers as the fibers pass proximate the second roller.
The step of transversely relatively moving set for above may comprises utilizing a controller to generate control signals according to a desired waveform for the fibers, feeding the control signals to a device for relatively moving the at least one of the rollers with respect to the other of the rollers in the second direction to thereby cause the shape of the fibers to conform to the desired waveform. The controller may comprise an electronic controller and the control signals may comprise electronic control signals which generate a waveform varying in at least one of a period, amplitude or shape characteristic. The step of applying the resin material may include spraying a resinous material onto the fibers or passing a support member carrying resinous material on at least one side of the fibers prior to the fibers making contact with the second roller to thereby impregnate the fibers with the resinous material, and subsequently separating the support member from the fibers.
Another embodiment of the invention may be characterized as a method of fabricating a material comprising the steps of a. passing a plurality of fibers to at least one first roller, the first roller contacting the plurality of fibers for moving same along a first direction, b. passing the plurality of fibers to at least one second roller, the second roller contacting the plurality of fibers for moving same along the first direction, c. transversely relatively moving at least one of the first and second roller in a second direction while each of the first and second rollers contacts the plurality of fibers thereby moving the plurality of fibers generally perpendicular to the first direction, d. applying a resin matrix to the fibers as the fibers pass proximate the second roller, and e. positioning a viscoelastic material at least adjacent the fibers. The positioning of the viscoelastic material may include commingling the viscoelastic material with the fibers and may be performed after the resin matrix applying step. The positioning step may include applying the viscoelastic material to the fibers at the same time as applying the resin matrix.
Transversely relatively moving the rollers may be achieved by utilizing an electronic controller to generate control signals according to a desired waveform for the fibers, and feeding the control signals to a device for relatively moving the at least one of the rollers with respect to the other of the rollers in the second direction to thereby cause the shape of the fibers to conform to the desired waveform.
Yet another embodiment of the invention is characterized as tape comprising a fiber tow, and a viscoelastic material containing the fiber tow. The viscoelastic material may be uncured or cured. Typically, the tow is orientated in substantially a straight line parallel to the direction of longitudinal axis of the tape. However, the tow may also form a sinusoidal waveform or a waveform that varies in one of a period, amplitude or shape characteristic along the longitudinal axis of the tape.
Yet another embodiment of the invention may be characterized as a composite structure comprising a first fiber tow, a resin matrix containing the fiber tow, a second fiber tow, a viscoelastic material containing the second fiber tow, the resin and viscoelastic material positioned adjacent one another. Typically at least one of the fiber tow of the matrix and the fiber tow of viscoelastic material has a waveform characteristic selected to dampen the at least one vibrational mode of a support structure. The fiber tow of the matrix may have a first waveform characteristic and the fiber tow of the viscoelastic material may have a second waveform characteristic, different from the first waveform characteristic. The first and second waveform characteristics may have opposite phases from one another. Further, at least one waveform may have a waveform characteristic variable in at least one of a period, amplitude or shape characteristic along the longitudinal direction of the composite structure.
In use, the composite structure comprising the resin and viscoelastic material are positioned adjacent an isotropic structure, the isotropic structure forming at least a part of a support structure. The composite structure dampens at least one vibrational mode of the support structure.
Yet another embodiment of the invention may be characterized as a method of applying a strengthening layer to a workpiece comprising the steps of wrapping a fiber tow about at least a portion of the workpiece, the fiber tow serving a strengthening layer, and subsequently applying a fiber re-enforced viscoelastic tape about at least the portion to secure the fiber tow to the workpiece.
Another embodiment of the invention may be characterized as a method of applying a strengthening layer to a workpiece comprising the steps of wrapping a fiber tow about at least a portion of the workpiece, the fiber tow serving a strengthening layer, and subsequently securing the fiber tow to at least the portion of the workpiece, wherein the step of wrapping the fiber tow includes the step of applying the fiber tow to the workpiece so as to take the shape of a desired non-spiral waveform. The step of securing the fiber tow may include applying a viscoelastic tape to the portion of the workpiece or applying a resinous material to the portion of the workpiece. Further, the applying step may include applying the fiber tow to the workpiece so as to take the shape of a sinusoidal waveform.
Yet another embodiment of the invention may be characterized as a method of applying a strengthening layer to a workpiece comprising the steps of wrapping a fiber tow about at least a portion of the workpiece, the fiber tow serving a strengthening layer, and subsequently securing the fiber tow to at least the portion of the workpiece, wherein the step of wrapping the fiber tow includes the step of applying the fiber tow to the workpiece so as to have a waveform characteristic varying in at least one of a period, amplitude or shape characteristic.
Yet another embodiment of the invention may be characterized as an apparatus for controlling the fiber angle of a fiber tow embedded in a carrier comprising:
a first pair of rollers including a first and second roller,
at least a third roller,
the fiber tow pressed between the first and second rollers and contacting the third roller for movement along a first direction due to rotation of the first, second and third rollers, a first drive mechanism for rotating the first, second and third rollers,
a second drive mechanism connected to at least one of the first pair of rollers and the third roller, the second drive mechanism responsive to control signals for displacing the at least one of the first pair of rollers and the third roller axially along the rotation axis of the at least one of the first pair of rollers and the third roller, and
an electronic controller connected to the second drive mechanism for generating the control signals to control the amount of axial displacement of the at least one of the first pair of rollers and the third roller.
The electronic controller may comprise a digital computer programmed to generate the control signals as a function of at least one of distance along the first direction and time. Further, the carrier may comprises a matrix material in the form of a rolled sheet and the apparatus further comprises. at least one support roller for securing the matrix material, the at least one support roller positioned for feeding the matrix material for contact with the third roller together with the fiber tow for at least partially securing the fiber tow in place within the matrix material.
The apparatus may further comprise:
a fourth roller positioned adjacent the third roller to form a second pair of rollers,
the fiber tow passing between the third and fourth rollers,
at least a first and second support roller and a first and second sheet of matrix material,
the first support roller supporting the first sheet of matrix material and the second support roller supporting the second sheet of matrix material, and
the first support roller positioned for feeding the matrix material of the first sheet between one of the third and fourth rollers and the fiber tow, and the second support roller positioned for passing the matrix material of the second sheet between the other of the third and fourth rollers and the fiber tow.
The matrix material may comprises an uncured polymer, a metal matrix or a ceramic matrix.
The apparatus may further comprise heating elements downstream of the second pair of rollers, in reference to the first direction, for causing wicking of the fibers with an uncured polymer matrix.
The carrier may comprise a viscoelastic material in the form of a rolled sheet and the apparatus may further comprise at least one support roller for securing the viscoelastic material, the at least one support roller positioned for feeding the viscoelastic material for contact with the third roller together with the fiber tow.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.